1. Field of the Invention
This invention relates to a sheet having a thickness of about 20 .mu. to 2 mm and having a spherulite size controlled to less than about 10 .mu. which is prepared by melt-molding a polyvinylidene fluoride or polymonochlorotrifluoroethylene resin. The sheet according to the present invention is a sturdy radiation transmitting material which transmits more than about 40% of the so-called dorno rays in a wavelength region of about 2,800 to 3,300A and is useful as a light admitting plate for a sun-room to admit the physiologically active rays in sunlight.
2. Description of the Prior Art
Ultraviolet rays having a wavelength in the region of 2800 to 3300A contained in sun radiation, i.e., the so-called dorno rays, are well-known to have an important physiological effect to convert ergosterin into vitamin D.sub.2 upon exposure to physiologically active rays in sunlight. When severe rachitis has occurred in many localities in the world, exposure to physiologically active rays in sunlight has hitherto been advocated by pediatricians since ultraviolet rays are very helpful for prevention and treatment of rachitis, and therefore, exposure to physiologically active rays in sunlight for therapeutic purposes has been used world-wide for babies and infants before the development of vitamin D.sub.2 synthesis. However, in recent years, dairy products containing vitamin D.sub.2 additives have become easily available and only administration of powdered milk or cod-liver oil containing a large quantity of vitamin D.sub.2 to babies and infants as a health measure has been considered sufficient to prevent rachitis without exposure to physiologically active rays in sunlight.
However, irregular mass examination in agricultural districts and a mountainous areas where babies and infants are usually fed on mother's milk has revealed that there is a tendency for the requirement of vitamin D.sub.2 to not be met. A noteworthy tendency which has been recently observed is that many babies and infants in overpopulated industrial cities, particularly those who are brought up by a dual-income family and who live in a small room in an apartment house suffer from a light rachitis condition. Further, it is reported that, as a recent abnormal phenomenon, babies and infants who are brought up in an airconditioned room frequently lack exposure to physiologically active rays in sunlight, even in summer, thereby resulting in an increase in rachitis patients which has not previously been considered to be a problem in the summer.
That is, rachitis problems which have previously been limited to small villages without health care programs, remote villages in high snowfall areas and agricultural areas and other areas during the winter season due to with reduced occurrence of ultraviolet rays, have now been occurring more generally throughout the country and over the entire year.
A requisite amount of vitamin D.sub.2 for babies and infants is believed to be about 400 to 800 IU per day. Nevertheless, it is reported in various parts of the country that an erroneous administration of vitamin D.sub.2 in a dose level higher than 10,000 IU per day for a prolonged period of time resulted in death due to hypervitaminosis and that an administration of vitamin D.sub.2 in a dose level of 1,800 IU per day, which is close to the requisite amount, retarded the normal growth of babies and infants or caused an abnormal hypercalcemia. These facts prove that an excessive intake of vitamin D.sub.2 at a dose level of more than 2,000 IU, even at a level of 1,500 IU, would be very dangerous for babies and infants. Hence, mothers, even physicians, have come to realize that the safety margin between the amount of vitamin D.sub.2 which is effective and the amount of vitamin D.sub.2 which is toxic is very narrow and that oral administration of vitamin D.sub.2 as a health measure may be hazardous to human health.
Since glass, clothes and the like scarcely transmit ultraviolet rays, many mothers have the burden of directly exposing their baby's body to physiologically active rays in sunlight for more than 30 minutes in the winter at which time dorno rays are very weak. Therefore, many proposals have been made to provide a material which sufficiently transmits dorno rays to allow indoor exposure to physiologically active rays in sunlight.
The results of measurement of monthly or hourly changes in solar radiation show that the amount of ultraviolet rays is poor over the period of from October to February. Exposure to physiologically active rays in sunlight is therefore necessary particularly during this period of the year. As far as dorno rays which have a wavelength of 2,800 to 3,300A and exhibit a physiological effect are concerned, the results of measurement of hourly changes in solar radiation show that the amount of energy reaches its maximum at 11 a.m. and the amount of energy obtained at 10 a.m. and between noon and 1 p.m. fall to half of the maximum value. Further, the results show that the amounts of radiation energy at 9 a.m. and 2 p.m. are almost equal corresponding to about 1/3 of the maximum value at 11 a.m. Accordingly, on exposure to physiologically active rays in sunlight during the fall and winter when the ultraviolet rays supplied are weak, the effective time for exposure is from about 9 a.m. to 2 p.m. while dorno rays after 3 p.m. become very weak and have such a low amount of energy that any health benefit is non-existant.
When physiologically active rays in sunlight passes through a high molecular weight material, it is assumed that the amount of the energy passed therethrough is decreased to only about 40% of the total amount of energy. This means that the maximum energy amount during the day (at 11 a.m.) falls to the level obtained on direct exposure at 8 a.m. to 9 a.m. or after 3 p.m. and that one cannot expect any beneficial effect from exposure to physiologically active rays in sunlight. Naturally, dorno rays are not only necessary for humans but also for animals and plants.
High molecular weight materials which permit the transmission of more than about 40% of dorno rays include polyolefins such as polyethylene, polypropylene and the like. However, the mechanical properties of these conventional materials are degraded markedly upon exposure to physiologically active rays in sunlight for several months and also the percent transmission of wavelengths of about 2,800 to 3,000A is decreased greatly due to the formation of a carbonyl group or a hydroxyl group arising from the splitting of the polyolefins main chains. Therefore, these materials are not considered to be suitable as a construction material for permanent facilities used for a long period of time.
Although anti-oxidants are generally incorporated into polyolefins as additives for the purpose of preventing the deterioration observed on standing, all of these additives have a property of absorbing ultraviolet rays and therefore, polyolefin materials having an improved weather-resistance are not useful where transmission of ultraviolet light, particularly dorno rays, is desired.
Polyfluorocarbon resins and acrylic resins are also known as high molecular weight materials. However, polytetrafluoroethylene has a high molding temperature which renders impossible the preparation of a strong and uniform sheet which can transmit more than about 40% of dorno rays. Polyvinyl fluoride and polymethyl methacrylate resins are considered to have good weather-resistance. However, not only do the mechanical properties of these materials deteriorate upon exposure to physiologically active rays in sunlight for a long period of time but also the percent transmission of dorno rays when used for a prolonged time decreases as compared with polyvinylidene fluoride. Particularly, polymethylmethacrylate is considered to have an extremely excellent transparency but this transparency can only be observed with visible rays and this material absorbs rays having a wavelength of 2,800 to 3,000 A so that it is not useful where transparency to rays having a wavelength corresponding to dorno rays is desired.
Polyvinylidene fluoride or polymonochlorotrifluoroethylene has excellent weather-resistance and can be easily molded to prepare a uniform sheet. However, those resin sheets having a spherulite size larger than about 10 .mu. which are prepared from these materials in a usual manner do not possess sufficient mechanical properties to resist wind pressure and snow load when they are exposed to cold winds or used in a snowy area, although they do show about 60 to 80% transmission of dorno rays when they are sufficiently thin, for example, less than about 20 .mu.. Therefore, these sheets are unsuitable for use as a light admitting plate of a sun-room. In addition, if the thickness of the sheet is greater than about 20 .mu., the percent transmission of dorno rays is drastically decreased.
Methods for molding thermoplastic resin sheets generally include the melt-extrusion method and the casting method using a solvent. In the casting method, it is difficult to remove a trace amount of the solvent remaining in the product and the percent transmission of dorno rays is greatly decreased due to even an extremely small amount of the remaining solvent.